Scientists had also noticed that promiscuous voles have fewer vasopressin (V1a) receptors, in a bit of their forebrain called the ventral pallidum region.

To prove vasopressin has a "taming" effect, the researchers gave meadow voles extra V1a receptors in the ventral pallidum region of their brains.

Reward system

The results were remarkable. After the V1a receptor gene was introduced, the former playboys reformed their ways.

Suddenly, they fixated on one female, choosing to mate with only her - even when other females tried to tempt them.

How does one hormone have such a dramatic effect? Scientists put it down to a particular chain of events.

They think that when the voles have sex, the hormone vasopressin is released. This hormone is then "picked up" by the V1a receptors in the ventral forebrain, which in turn trigger a neural "reward system".

The reward system makes them feel happy, and they associate those feelings with the vole they have just mated with - which encourages them to stick around.

"We think what happens is when the voles mate, vasopressin activates the reward centre, and it really makes the animals pay attention to who they are mating with," co-author Larry Young, from Emory University, Georgia, US, told BBC News Online.

He continued: "It makes the voles think, 'when I'm with this partner I feel good'. And from then on, they want to spend their time with that particular partner."

The researchers have concluded that the male meadow voles are promiscuous because they lack just one link in the chain: the V1a receptors in the ventral forebrain.

Autism link?

The implications of this study extend beyond Casanova voles, however. The strings of human behaviour might be pulled by similar hormones and similar pathways.

"We know that vasopressin is released when humans have sex," said Professor Young. "Sex is probably involved in maintaining the bond between humans and vasopressin may play a role in that."

Jealous wives might want to give their husbands a hefty dose of the V1a receptor gene, but Professor Young and his team are focussed on medical advances.

Studies of this kind could, they say, open the lid on conditions where our social skills go wrong, such as in autism.

The strings of human behaviour might be pulled by similar hormones and pathways

"Part of the reason we are doing this research is that we are trying to understand the social brain," explained Professor Young. "Why do we interact with other people, and what could be wrong in diseases like autism?

"In autism, people are very aloof - they don't want to interact with others. It could be that vasopressin plays a role in normal human social interactions.

"Two studies have already found there is a modest link between vasopressin and autism."

Professor Joseph Piven, a psychiatry expert from the University of North Carolina at Chapel Hill, US, agrees the voles might reveal something interesting about autism.

"This is a model where the voles have alterations in their social behaviours," he said. "And these alterations may be linked to the same processes that are going awry in autism."

He continued: "No very strong links have been found yet between autism and vasopressin, but after studies like this people may take a closer look."